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In Arkham Horror, an investigator must pay the sanity cost of a spell before making a spell check to see whether it would succeed. If the spell's sanity cost is X, and the investigator has only X sanity, can that investigator willfully attempt to cast the spell in order to go insane?

For example, Gloria has only 1 sanity, and she needs to reach the other side of Arkham all the way from St. Mary's Hospital, but moving there on foot will take her 5 turns due to some ongoing Mythos environment. She has the spell Withering, which has a sanity cost of 1. Even though she does not have the lore to cast it successfully, can she use that spell to go insane and teleport herself all the way to Arkham Asylum?

To recap: Spells cannot be used unless they can be paid, but can you game the system when the situation demands it such that you obtain the benefits of paying everything you have?

2 Answers
2

Q: When an investigator casts a spell, can he spend all of his Sanity
to pay the spell’s Sanity cost?

A: Yes. If he does so, once the Sanity
cost has been paid the investigator makes his Spell check and, if he
passes, the spell takes effect. Whether the check is passed or not,
the investigator then goes insane. Note that if an investigator were
to cast a spell to gain combat bonuses, he would go insane before
having a chance to get into combat

Yes. Just as there's nothing stopping you from not using your weapons to purposefully fail a combat check against a Nightgaunt to go through a gate, or purposefully failing a sneak check to take combat damage and get sent to the hospital, there's nothing stopping you from casting a spell when there's no chance of you succeeding. The closest thing I can come up with for a reference to your suggested scenario is a question from the Kevin Wilson Answers thread on the Fantasy Flight Games forum:

If one goes insane or unconscious or is devoured as a result of casting a spell or using an item, is it generally the case that the
effect of the spell or item still occurs (in the same way an Elder
Sign's effect takes place even if one of the above happens as a result
of using it)?

Yes. It will often not matter, however, as in the case where you're
getting a combat bonus or something.

So it's definitely allowed that you could cast a spell that drives you insane in the process. So a useful last-sanity draining spell use could be Healing another investigator, or casting Bind Monster, or Greater Banishment, etc. Anything with an immediate effect after making the Casting Check. The only limitation on the spell casting I'd say is that you can't try to cast a spell outside of a situation it would be useful in. i.e. I wouldn't allow casting Shriveling outside of combat just so the Investigator could go insane.